1. Field
The present disclosure relates to timing adjustments for channel estimation in a multi carrier wireless system, and, more particularly, to adjusting timing by ensuring pilot tone interlaces have matching time bases, which also match a symbol time basis.
2. Background
Orthogonal frequency division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different carrier frequencies orthogonal to one another. These channels are sometimes called subbands or subcarriers. In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM) except in the way in which the signals are modulated and demodulated. One advantage of OFDM technology is that it reduces the amount of interference or crosstalk among channels and symbols in signal transmissions. Time-variant and frequency selective fading channels, however, present problems in many OFDM systems.
In order to account for time varying and frequency selective fading channels, channel estimation is used. In coherent detection systems, reference values or “pilot symbols” (also referred to simply as “pilots”) embedded in the data of each OFDM symbol may be used for channel estimation. Time and frequency tracking may be achieved using the pilots in channel estimation. For example, if each OFDM symbol consists of N number of subcarriers and P number of pilots, then an N-P number of the subcarriers can be used for data transmission and P number of them can be assigned to pilot tones. These P number of pilots are sometimes uniformly spread over the N subcarriers, so that each two pilot tones are separated by N/P-1 data subcarriers (or, in other words, each pilot occurs every N/Pth carrier). Such uniform subsets of subcarriers within an OFDM symbol and over a number of symbols occurring in time are called interlaces.
In one area of application, OFDM has also been used in Europe and Japan, as examples, for digital broadcast services, such as with the Digital Video Broadcast (DVB-T/H (terrestrial/handheld)) and Integrated Service Digital Broadcast (ISDB-T) standards. In such wireless communication systems, channel characteristics in terms of the number of channel taps (i.e., the number of samples or “length” of a Finite Impulse Response (FIR) filter that is used to represent the channel of a received signal) with significant energy, path gains, and the path delays are expected to vary quite significantly over a period of time. In an OFDM system, a receiver responds to changes in the channel profile by selecting the OFDM symbol boundary appropriately (i.e., correction of window timing) to maximize the energy captured in a fast Fourier transform (FFT) window.
When timing corrections take place, it is important that the channel estimation algorithm takes the timing corrections into account while computing the channel estimate to be used for demodulating a given OFDM symbol. In some implementations, the channel estimate is also used to determine timing adjustment to the symbol boundary that needs to be applied to future symbols, thus resulting in a subtle interplay between timing corrections that have already been introduced and the timing corrections that will be determined for the future symbols. Further, it is common for a channel estimation block in a receiver to buffer and then process pilot observations from multiple OFDM symbols, which results in a channel estimate that has better noise averaging and resolves longer channel delay spreads. This is achieved by combining the channel observations from consecutively timed OFDM symbols into a longer channel estimate in a unit called the time filtering unit. Longer channel estimates in general may lead to more robust timing synchronization algorithms. When pilot observations from multiple OFDM symbols are processed together to generate a channel estimate, however, if the interlaces combined and the OFDM symbols to be demodulated are not aligned with respect to the symbol timing (i.e., have the same time-basis), the channel estimation may become degraded to the point that it cannot be used for successful symbol demodulation.